Field weakening control method in induction motor

ABSTRACT

A field weakening control method in an induction motor, wherein a voltage limit is set up in an oval while a current limit is established in a circle on a current plane, where x-coordinate represents a current value for flux portion and y-coordinate defines a current value for torque portion when the motor is in a normal state of constant speed and no load applied, to thereby form a current command according to a juncture thereof for generation of a torque, such that the oval voltage limit moves to a predetermined -(minus) quadrant against a coordinate representing the current value for torque portion in an excessive state where accelerated speed is formed or load is applied, to thereby cause torque of excessive state to increase in comparison with the normal state, such that there is an advantage in that torque is increased to improve an accelerated speed efficiency when the induction motor is under excessive state, and field weakening is conventionally controlled when the induction motor is under a normal state, thereby improving an efficiency during the excessive state and overall torque of the induction motor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a control method in an induction motorand more particularly to a field weakening control method in inductionmotor for controlling reverse electromotive force of an induction motor.

2. Description of the Prior Art

Generally, induction motors generate reverse electromotive force whenthey are operated according to rotating speed.

When the reverse electromotive force is increased in proportion to arotary speed, there sometimes occurs a case where the reverseelectromotive force becomes greater than driving voltage applied to themotor, such that in order to avoid this kind of occurrence, a fieldweakening control is enforced to decrease the flux in reverse proportionto rotating speed of the motor. In other words, decrease of the flux inreverse proportion to the rotating speed of the motor serves to decreasetorque and to prevent the reverse electromotive force from growinglarger than the driving voltage.

However, when the flux is decreased in reverse proportion to rotatingspeed, there occurs too much loss of driving torque at a high speedregion, such that field weakening region is controlled as below in orderto maximize the driving torque at the high speed region. Rotor flux ofan induction motor in the control of field weakening region is shown asper the following formula 1.

    λ.sub.dr.sup.e =L.sub.m i.sub.ds.sup.e              [Formula 1]

where,

λ_(dr) ^(e) : rotor flux

L_(m) : magnetization inductance and

i_(ds) ^(e) : current for synchronized coordinate flux

The rotor flux according to Formula 1 can be obtained by simultaneoussolutions according to voltage and current limiting condition ofinvertor determined by Formula 2 and stator simultaneous coordinatevoltage equation according to Formula 3.

    V.sub.ds.sup.e2 +V.sub.qs.sup.e2 ≦V.sub.s.sup.2.sub.max

    i.sub.ds.sup.e2 +i.sub.qs.sup.e2 ≦I.sub.s.sup.2.sub.max,[Formula 2]

where,

i_(ds) ^(e) : current for synchronous coordinate flux

i_(qs) ^(e) : current for synchronous coordinate torque

V_(ds) ^(e) : voltage for synchronous coordinate flux, and

V_(qs) ^(e) : voltage for synchronous coordinate torque.

V_(s) max which is a voltage limiting value in the above formula 2 isdefined by general inverter direct current voltage and voltagemodulating method, and I_(s) max which is a current limiting value isobtained by semiconductor element thermal rating and current rating.Here, if a rotor flux of an induction motor is operated at a normalcondition, an equation for obtaining an induction motor rotorsynchronous coordinate voltage can be given by the following formula 3.

    V.sub.qs.sup.e =r.sub.s i.sub.ds.sup.e +ω.sub.e L.sub.s i.sub.ds.sup.e

    V.sub.ds.sup.e =r.sub.s i.sub.ds.sup.e -ω.sub.e L.sub.σ i.sub.qs.sup.e                                            [Formula 3]

where,

r_(s) : stator resistance value

ω_(e) : revolution

L_(s) : stator inductance, and

L.sub.σ : leakage inductance

As mentioned above, when the Formulas 2 and 3 are united, a voltagelimit at current plane is expressed in an oval and a current limit isdefined in a circle, as illustrated in FIG. 3, where, an inner jointregion of the oval and the circle represents a current commandcorresponding to an operational condition.

Of course, the above voltage equation of stator synchronous coordinateis given under an resumption that rotor flux is in normal state (namely,speed and load are under a constant state). Rotor flux and current fortorque portion at a field weakening region No. 1 where a juncturebetween the oval and the circle is formed are expressed by Formula 4 androtor flux and current for torque portion at a field weakening regionNo. 2 where a juncture between the oval and the circle is not formed(super high speed condition) are defined by Formula 5. ##EQU1##

In other words, the above weakened field control method for inductionmotor assumes that a motor is in a normal operational state, where samereverse electromotive force is decreased from the entire motoroperational regions according to Formulas 4 and 5, thereby expressingthe rotor flux and current for torque portion.

Here, speed arrived characteristic experimental value of the inductionmotor is (1-X) which is the time reaching 4,400 rpm as illustrated inFIG. 4, where torque and flux from predetermined curves.

SUMMARY OF THE INVENTION

However, there is a problem in that, when the field weakening control isperformed as mentioned above on the assumption that the operationalstate of the induction motor is in the normal condition, torque in anexcessive state (when load is applied and when speed is increased ordecreased) of the induction motor becomes very weak, such that anaccelerated characteristic of the induction motor is deteriorated tothereby decrease the entire efficiency of the induction motor.

The present invention is disclosed to solve the aforementioned problemand it is an object of the present invention to provide a fieldweakening control method in induction motor adapted to improve anaccelerated characteristic of an induction motor in an excessive state,thereby increasing an entire efficiency of the induction motor.

In accordance with the object of the present invention, there isprovided a field weakening control method in an induction motor, whereina voltage limit is set up in an oval while a current limit isestablished in a circle on a current plane, where x-coordinaterepresents a current value for flux portion and y-coordinate defines acurrent value for torque portion when the motor is in a normal state ofconstant speed and no load applied, to thereby form a current commandaccording to a juncture thereof for generation of a torque, such thatthe oval voltage limit moves to a predetermined minus quadrant against acoordinate representing the current value for torque portion in anexcessive state where accelerated speed is formed or load is applied, tothereby cause torque of excessive state to increase in comparison withthe normal state.

BRIEF DESCRIPTION OF THE DRAWINGS

For fuller understanding of the nature and objects of the invention,reference should be made to the following detailed description taken inconjunction with the accompanying drawings in which:

FIG. 1 is a graph for illustrating a current limit and a voltage limiton a current plane of excessive state in a field weakening controlmethod according to the present invention;

FIG. 2 is a graph for illustrating a speed arrival characteristicexperimental value of an induction motor utilizing a field weakeningcontrol method according to the present invention;

FIG. 3 is a graph for illustrating current and voltage limit on acurrent plane during a normal state and accelerated/decelerated speed ina field weakening control method in a general induction motor; and

FIG. 4 is a graph for illustrating a speed arrival characteristic of aninduction motor in FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a graph for illustrating a current limit and a voltage limiton a current plane of excessive state in a weakened field control methodaccording to the present invention, where it is shown that a voltagelimit region defined by an oval against a current limit represented by acircle is moved by -(minus) δ against the current for torque portion.

Rotor flux according to the graph in FIG. 1 is given by Formula 6 below.##EQU2## p: differential operator τ_(r) : rotor time constant

Of course, flux under a normal state of induction motor is controlledaccording to Formulas 4 and 5, however a weakened field controlaccording to Formula 6 is applied only when the induction motor is underexcessive state (during application of load or accelerated/deceleratedspeed).

A synchronous coordinate stator voltage equations according to theexcessive state characteristic of rotor flux are expressed by Formula 7below. ##EQU3##

When Formula 7 is solved in association with Formula 3, a voltage limitoval which shows a movement of -(minus) δ is formed as shown in FIG. 1,and when a junction between the voltage limit oval and current limitcircle is obtained, a current value for rotor flux portion and a currentvalue for torque portion which show a maximum torque at the weakenedfield region No. 1 can be obtained. This is defined in Formula 8 below.##EQU4## where, δ=L_(m) /L_(r) αe1/τ1)

Here, the reason the rotor flux of Formula 6 shows the maximum torqueaccording to Formula 8 is that the voltage limit oval moves by -(minus)δ during excessive state to cause the torque according to the currentvalue of Formula 8 to increase, thereby further expediting speedincrease during application of load or accelerated/decelerated speed.

In other words, as illustrated in FIG. 2, the time arriving at 4,400 rpmbecomes (1-X₁), where x₁ >x and difference between x₁ and x isapproximately 10˜20%, and arrival time of the present invention isexpedited as much as the difference to thereby increase an operationalefficiency of the entire motor.

A value according to field weakening region No. 2 where a junctionbetween the current limit circle and the voltage limit oval does notoccur is expressed by Formula 9 below. ##EQU5##

As apparent from the foregoing, there is an advantage in the fieldweakening control method according to the present invention in thattorque is increased to improve an accelerated speed efficiency when aninduction motor is under excessive state, and field weakening isconventionally controlled when the induction motor is under a normalstate, thereby improving an efficiency during the excessive state andoverall torque of the induction motor.

What is claimed is:
 1. A field weakening control method in an inductionmotor, wherein a voltage limit is set up in an oval while a currentlimit is established in a circle on a current plane, where x-coordinaterepresents a current value for flux portion and y-coordinate defines acurrent value for torque portion when the motor is in a normal state ofconstant speed and no load applied, to thereby form a current commandaccording to a juncture thereof for generation of a torque, such thatthe oval voltage limit moves to a predetermined minus quadrant against acoordinate representing the current value for torque portion in anexcessive state where accelerated speed is formed or load is applied, tothereby cause torque of excessive state to increase in comparison withthe normal state.
 2. The control method as defined in claim 1, wherein acurrent value for rotor flux portion and a current value for torqueportion in a region where said juncture is formed between voltage limitvalue and current limit value in excessive state, are determined usingformulas as follows: ##EQU6## where:

    δ=L.sub.m /L.sub.r αe.sup.1/τ1

    i.sub.qs.sup.e =√I.sub.s.sup.2.sub.max -i.sub.ds.sup.e2

i_(ds) ^(e) =current for synchronous coordinate flux i_(qs) ^(e)=current for synchronous coordinate torque L_(s) =stator self inductanceL.sub.σ =leakage inductance L_(m) =magnetizing inductance L_(r) =rotorself inductance ω_(e) =excitation angular frequency.
 3. The controlmethod as defined in claim 1, wherein a current value for rotor torqueportion and a current value for flux portion in a region where saidjuncture is not formed between the oval voltage limit and circledcurrent limit, are determined by using formulas as follows: ##EQU7##i_(ds) ^(e) =current for synchronous coordinate flux i_(qs) ^(e)=current for synchronous coordinate torqueL_(s) =stator self inductanceL_(m) =magnetizing inductance ω_(e) =excitation angular frequency.